Lint str-ptr-in-c-abi discourage str pointers in C ABI fns by fpdotmonkey · Pull Request #17401 · rust-lang/rust-clippy

fpdotmonkey · 2026-07-10T18:43:07Z

Please write a short comment explaining your change (or "none" for internal only changes)

changelog: str-ptr-in-c-abi: This discourages using a pointer to a str instead of to a CString for extern "C" functions.

This addresses #1236

rustbot · 2026-07-10T18:43:11Z

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fpdotmonkey · 2026-07-10T20:32:19Z

I'm not really sure what the failure on Lintcheck is. The error messages sounds like failures in external crates, but that's almost certainly not the case.

Gri-ffin · 2026-07-10T21:09:54Z

You are currently using fn_sig unconditionally, you should check if it's an FnDef or FnPtr.
@rustbot author

rustbot · 2026-07-10T21:13:22Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

github-actions · 2026-07-10T21:45:34Z

Lintcheck changes for 9669223

Lint	Added	Removed	Changed
`clippy::str_ptr_in_c_abi`	1	0	0

This comment will be updated if you push new changes

fpdotmonkey · 2026-07-10T21:47:45Z

You are currently using fn_sig unconditionally, you should check if it's an FnDef or FnPtr.

Fixed

@rustbot ready

This discourages using a pointer to a `str` instead of to a CString for `extern "C"` functions.

rustbot · 2026-07-11T11:21:03Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Gri-ffin

@rustbot author

View changes since this review

Gri-ffin · 2026-07-11T11:55:51Z

+        let ExprKind::Call(callee, args) = expr.kind else {
+            return;
+        };
+        // where the signature of the callee shows it is
+        // extern "C"
+        if is_extern_c_fn(cx, callee) {
+            // and takes a raw pointer as an argument
+            let arg_idxs = raw_pointer_arg_idxs(cx, callee);
+            if arg_idxs.is_empty() {
+                return;
+            }
+            let relevant_args = arg_idxs
+                .into_iter()
+                .filter_map(|idx| args.get(idx))
+                .map(Clone::clone)
+                .collect::<Vec<_>>();
+            let problem_args = args_str_ptr_cast(cx, &relevant_args);
+            if problem_args.is_empty() {
+                return;
+            }
+            let span: rustc_errors::MultiSpan = problem_args
+                .into_iter()
+                .map(|arg| arg.span)
+                .collect::<Vec<rustc_span::Span>>()
+                .into();


You can combine these checks into an if let chain and use an iterator that only collect at the end. (See review below). then you can pass the spans directly to span_lint_and_help (if they are not empty).

Gri-ffin · 2026-07-11T12:10:08Z

+/// Returns elements from `args` which have a Rust `str` as a raw pointer cast to a raw pointer.
+fn args_str_ptr_cast<'tcx>(cx: &LateContext<'tcx>, args: &[Expr<'tcx>]) -> Vec<Expr<'tcx>> {
+    args.iter()
+        .filter(|arg| {
+            // just a simple check for
+            // `let s: &str; libc::strlen(s.as_ptr() as *const _);`
+            // or similarly with `as_mut_ptr`/`*mut`
+            if let ExprKind::Cast(expr, ty) = arg.kind
+                && matches!(ty.kind, TyKind::Ptr(_))
+                && let ExprKind::MethodCall(method, this, _args, _span) = expr.kind
+                && ["as_ptr", "as_mut_ptr"].contains(&method.ident.name.as_str())
+                && is_ref_str(cx.typeck_results().expr_ty(this))
+            {
+                true
+            } else {
+                false
+            }
+        })
+        .map(Clone::clone)
+        .collect()
+}
+
+/// is `ty` either `&str` or `&mut str`?
+fn is_ref_str(ty: Ty<'_>) -> bool {
+    if let rustc_middle::ty::Ref(_, reffed_ty, _) = ty.kind() {
+        return reffed_ty.is_str();
+    }
+    false
+}


You can replace this with a single function that checks one argument and returns a bool if it's cast to a raw pointer (for example a is_str_ptr_cast that you can use in the above filter closure in the iterator), you can use expr_ty_adjusted to catch String, etc... instead of being limited to str.
Also you can use sym from clippy_utils instead of comparing strings directly, like this:

.... matches!(method.ident.name, sym::as_ptr | sym::as_mut_ptr)

rustbot added the S-waiting-on-community-reviews Status: This is awaiting for positive reviews from the community before a maintainer is assigned. label Jul 10, 2026

rustbot added needs-fcp PRs that add, remove, or rename lints and need an FCP S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jul 10, 2026

fpdotmonkey force-pushed the lint-str-ptr-in-c-abi branch 3 times, most recently from db505ce to 0d5dddd Compare July 10, 2026 20:06

rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jul 10, 2026

fpdotmonkey force-pushed the lint-str-ptr-in-c-abi branch from 0d5dddd to 5f74d2d Compare July 10, 2026 21:36

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties and removed S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) labels Jul 10, 2026

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Lint str-ptr-in-c-abi discourage str pointers in C ABI fns

9669223

This discourages using a pointer to a `str` instead of to a CString for `extern "C"` functions.

fpdotmonkey force-pushed the lint-str-ptr-in-c-abi branch from 5f74d2d to 9669223 Compare July 11, 2026 11:21

Gri-ffin reviewed Jul 11, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action from the author. (Use `@rustbot ready` to update this status) and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties labels Jul 11, 2026

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Lint str-ptr-in-c-abi discourage str pointers in C ABI fns#17401

Lint str-ptr-in-c-abi discourage str pointers in C ABI fns#17401
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